This invention relates to automated blood pressure monitoring, and more particularly, to automated blood pressure monitors that use a signal quality measurement in determining whether a noninvasive blood pressure measurement using that signal is reliable.
Automated oscillometric blood pressure monitors are known in the art in which a curve is computationally fitted to the oscillometric envelope defined by the amplitudes of complexes at varying cuff pressures, thereby enabling mean arterial pressure (MAP) and systolic and diastolic blood pressures to be more accurately computed. As explained in U.S. Pat. No. 5,704,362, such curve fitting computations are inherently immune to aberrations caused by artifacts. Generally, such a technique calls for a Gaussian-shaped function to be computationally model fitted to the oscillometric envelope, although other functions could be used. The curve-fitting computations employ the Marquardt method which is a combination of the steepest descent on a sum-squared error function and Gauss-Newton zero-finding for an observation function. The method described in U.S. Pat. No. 5,704,362 constrains the envelope to the known reasonable shape of the Gaussian function, thereby providing a filtering method which makes the curve fitting less dependent upon any single data point. This allows artifact detection techniques during data gathering to be relaxed or eliminated. Also, the envelope data may include historical data collected over several blood pressure determinations and weight-averaged to provide weight-averaged prediction of the next blood pressure determination.
The Gaussian curve fitting method employed in U.S. Pat. No. 5,704,362 determines a set of three parameters based on data from the present or from previous blood pressure determinations, these parameters including the envelope amplitude (A), the mean (B), and the deviation from the mean (C). In other words, the Gaussian curve is defined by its amplitude, mean, and deviation, and the curve fit is defined as the curve with the amplitude, mean, and deviation which minimizes the sum-squared error (s.s.e.) between the Gaussian curve and the data points at each of the raw envelope pressures. A search is conducted in the (A,B,C) variable space until a minimization point is found. Data from the current blood pressure determination may be used to shift the Gaussian curve from a previous determination to the correct pressure vicinity so that it more closely fits the most recently measured data. This can help with identifying the amplitude of raw blood pressure complexes and rejecting artifact. Further details regarding the curve fitting method described in U.S. Pat. No. 5,704,362 are hereby incorporated by reference.
Unfortunately, even such curve fitting techniques must address the problem that patient motion, vibrations, and other interference may cause artifact in the pressure signal obtained from the cuff during the blood pressure determination. When this happens, identifying blood pressure complexes and their properties is troublesome, even when using the above-referenced curve-fitting techniques. It can then be difficult to decide when to publish blood pressure results and when to give warnings that artifact is present or that the output may be inaccurate. The present invention relates to systems and methods which have been developed to handle this problem.
The present invention addresses the afore-mentioned problems in the art by using objective criteria to determine the quality and reliability of the measured NIBP data prior to presenting the data to the monitor""s display for viewing. In accordance with the invention, the oscillometric envelope data is checked for shape, quality of curve fit (if a curve fit procedure is used), history quality, envelope quality, complex quality, and step quality. These quality values are then combined into an overall quality value that is used to determine whether or not to publish the gathered oscillometric envelope data and whether messages warning of artifacts should also be given.
In particular, the present invention relates to a method of measuring the blood pressure of a subject, comprising the steps of:
obtaining from the subject a plurality of oscillometric data values from an amplitude of at least one complex taken at a plurality of pressure levels, the oscillometric data values representing points of an oscillometric envelope defined by measured blood pressure oscillations;
calculating the patient""s blood pressure from the oscillometric data values;
checking the signal quality of the oscillometric data values; and
selectively displaying the calculated blood pressure in accordance with the signal quality of the oscillometric data values.
The method of the invention is implemented by an automated sphygmomanometer apparatus comprising an inflatable and deflatable pressure cuff, an inflating apparatus coupled to the cuff so as to selectively apply a medium under pressure to the cuff for inflating and pressurizing the cuff, a cuff pressure sensor coupled to the cuff so as to sense cuff pressure including any blood pressure oscillations therein, a deflating apparatus coupled to the cuff so as to selectively relieve pressure from the cuff, and a programmed control device responsive to a cuff pressure determination of the cuff pressure sensor. In accordance with a preferred embodiment of the invention, the control device is programmed to control the inflating apparatus to inflate the cuff and the deflating apparatus to deflate the cuff during respective blood pressure determinations of a patient at predetermined intervals and to store oscillometric envelope data representing points of an oscillometric envelope defined by measured blood pressure oscillations. Also, the control device is further programmed to calculate the patient""s blood pressure from the oscillometric envelope data, to check the signal quality of the oscillometric envelope data, and to selectively display the calculated blood pressure in accordance with the signal quality of the oscillometric envelope data.
The programmed control device checks the signal quality of the oscillometric envelope data in accordance with the invention by determining if the oscillometric envelope has a predetermined general bell shape, by using blood pressure results determined during implementation of a curve fit procedure (if used) to the oscillometric envelope data to determine if the calculated blood pressures are such that diastolic less than MAP less than systolic and in a reasonable physiological range, by comparing newly acquired oscillometric envelope data with stored oscillometric envelope data and determining an intermediate history quality number as a percentage of values of the newly acquired oscillometric envelope data that are within a predetermined range from values of the stored oscillometric envelope data, by determining an intermediate envelope quality number as a measure of how well curve fit data used by the curve fit procedure fits the newly acquired oscillometric envelope data, by determining an intermediate complex quality number as a measure of a percentage of pressure steps whose best complexes are above an estimated noise level that is a root mean square (r.m.s.) error of all complexes in the newly acquired oscillometric envelope data, and/or by determining an intermediate step quality number as a measure of the variability of sizes of complexes at an envelope step pressure level. Preferably, these values are combined in accordance with a weighting function to create an overall quality number representative of the signal quality of the oscillometric envelope data. Generally, more recent oscillometric envelope data is weighted more heavily than older oscillometric envelope data.
In a preferred embodiment, the programmed control device determines the intermediate envelope quality number using the equation:
Intermediate envelope quality=A*100/(A+sqrt(WEIGHT*Envelope s.s.e.))
where A is a Gaussian parameter for amplitude used by the curve fit procedure, WEIGHT has a value based on the intermediate history quality number, and envelope sum-squared error (s.s.e.) is found using the following equation:
Envelope s.s.e.=xcexa3(aixe2x88x92Axc2x7exe2x88x92(Pixe2x88x92B)2/C)2
where xe2x80x9caixe2x80x9d and xe2x80x9cpixe2x80x9d represent the oscillometric envelope data and correspond to step oscillation amplitude and step pressure, respectively, xe2x80x9cixe2x80x9d is an index used for envelope step data, and B and C are Gaussian parameters for mean, and deviation, respectively, used by the curve fit procedure. The programmed control device also bases the estimated noise level on a complex s.s.e. determined from the following equation:       complex    ⁢          xe2x80x83        ⁢          s      .      s      .      e      .        =            ∑              i        ,        j              ⁢                  (                              c            ij                    -                      A            ·                          ⅇ                                                -                                                            (                                                                        p                          i                                                -                        B                                            )                                        2                                                  /                C                                                    )            2      
where cij is complex data representing complex size from the newly acquired oscillometric envelope data, xe2x80x9cixe2x80x9d is an index used for envelope step data, j is an index for the complexes at an envelope step pressure level, pi represents step pressure, and A, B, and C are Gaussian parameters for amplitude, mean, and deviation, respectively, used by the curve fit procedure.
In the preferred embodiment, the programmed control device determines the intermediate step quality number as a percentage of complexes, out of all complexes received, which has a ratio (an absolute difference between each complex to a best estimate of complex size for the cuff pressure at which the complex occurs) which exceeds a threshold dependent upon the intermediate history quality number.
Preferably, the programmed control device checks the calculated blood pressure and the overall quality number to determine if the calculated blood pressure and the overall quality number make physiological sense prior to displaying the calculated blood pressure. Then, the programmed control device compares the overall quality number to a first threshold, whereby the oscillometric envelope data is displayed only if the first threshold is exceeded. The programmed control device may also compare the overall quality number to a second threshold, greater than the first threshold, whereby the oscillometric envelope data is displayed with a message warning of artifact if the overall quality number exceeds the first threshold but not the second threshold and displays the oscillometric envelope data without the warning message if the overall quality number exceeds both the first and second thresholds.
The scope of the invention also includes corresponding methods as apparent from the following detailed description of the invention.